Microwave amplifier utilizing multipaction to produce periodically bunched electrons



3,312,857 ION TO ONS 5 Sheets-Sheet 1 T. FARNSWORTH TILIZIN LLY BUN Apr1l4, 1967 p,

MICROWAVE AMPLIFIER U G MULTIPACT PRODUCE PERIODICA CHED ELECTR Filed April 19, 1965 INVENTOR. fh/o ZFZr/YsWar-Zk, Meg/2 1 (QJM wd'zt'arnegs.

A ril 4, 1967 P. "r. FARNSWORTH 3,312,357

MICROWAVE AMPLIFIER UTILIZING MULTIPACTION TO PRODUCE PERIODICALLY BUNCHED ELECTRONS 1963 5 Sheets-Sheet 2 Filed April 19,

Apnl 4. 19 P. T. FARNSWORTH 3,

MICROWAVE AMPLIFIER UTILIZING MULTIPACTION TO 7 PRODUCE PERIODICALLY BUNCHED ELECTRONS Filed April 19, 1963 5 Sheets-Sheet 3 4 v My. 5. I

Mtorweys p il 1967 P. T. FARNSWORTH 3,312,857

MICROWAVE AMPLIFIER UTILIZING MULTIPACTION TO PRODUCE PERIODICALLY BUNCHED ELECTRONS I Filed April 19, 1963 5 Sheets-Sheet 4;

wary/76 April 4, 1967 PRODUCE PER Filed April 19, 1963 P. 'r. FARNSWORTH 3,312,857 MICROWAVE AMPLIFIER UTILIZING MULTIPACTION TO IODICALLY BUNCHED ELECTRONS 5 Sheets-Sheet 5 United States Patent 3,312,857 MICROWAVE AMPLIFER UTILIZING MULTIPAC- TION TO PRODUCE PERIODICALLY BUNCHED ELECTRONS Philo T. Farnsworth, Fort Wayne, Ind., assignor to International Telephone and Telegraph Corporation, Nutley, N.J., a corporation of Maryland Filed Apr. 19, 1963, Ser. No. 274,128 31 Claims. (Cl. 315-5) The present invention relates to a microwave amplifier and more particularly to an amplifier wherein energy from a beam of bunched electrons is utilized in the generation of an amplified signal. Compared to a conventional klystron, this amplifier is more effieient, is smaller and lighter in weight, has longer operational life and is simpler to use.

In velocity modulation amplifiers, such as the klystron, an indirectly heated cathode furnishes a beam of electrons with a uniform average velocity. A radio frequency field between the grids of a buncher cavity in the klystron modifies the velocity of the electrons passing through to the extent that the electrons which have been speeded up by the buncher will overtake, in a drift space beyond the buncher, the electrons which left the buncher earlier but were slowed down during the previous half cycle. As a result, the beam current between the grids of the second or catcher resonator will be pulsating or bunched and if the second resonator is tuned to the bunching frequency, the beam will deliver power to the catcher.

More particularly, if high power output is required from a klystron, a correspondingly intense electron beam is needed and the thermionic cathode must have a large electron emitting surface with attendant high thermal radiation loss. Also, such tubes are usually operated as pulse amplifiers where the pulse amplitude may be 50 to 500 times the average amplitude. The thermionic emitter must be designed to .provide these very large peak currents. Thus, the thermionic cathode introduces formidable cooling problems. As in any thermionic device, the klystron requires a warm-up time to reach stable operation, and the operational life is usually limited by the cathode surface. After a period of non-use, a forming-in period of several hours is required.

Since in the klystron the average energy of the electrons leaving the resonator will be greater than those entering, the bunching resonator must supply a substantial amount of power to bunch the beam. Since the drift space in the klystron is. of course, fixed, and optimum bunching occurs for one value of excitation, the selection and maintenance of the proper excitation and acceleration voltages becomes quite critical. The space charge represented by the concentration of electrons in the bunches per se tends to destroy the bunches in the drift space, causing the beam to diverge. Both longitudinal and transverse debunching detract-s from the efiiciency of the klystron. Therefore, there must be a compromise in good klystron design between the gain possible by increasing the drift space and the loss due to debunch-ing. The maximum theoretical efficiency is fifty-eight percent (58%). Practical efficiencies are on the order of twenty percent (20%) because of various second-order bunching effects of the space charge forces. Much of this loss in the klystron is in the form of heat to be removed by external cooling devices. Secondary electrons from unwanted multip'acting in the catcher-collector region of the klystron also decrease the efiiciency and cause other undesirable effects.

Klystrons are also subject to undesirable frequency changes by thermal expansion and contraction with changes in temperature. The resonant circuit of the klystron is heated by power dissipated in the tube. These 3,312,857 Patented Apr. 4, 1967 tron discharge device for use in amplifying signals in the microwave spectrum.

Another object of this invention is to provide an ultra-high frequency amplifier wherein the generation and bunching of electrons are achieved in an efiicient, reliable and facile manner.

It is still another object of this invention to provide an ultra-high frequency amplifier which generates and bunches electrons emitted from a cold-cathode source by the process known as phase focusing.

It is yet another object to provide an ultra-high frequency amplifier which requires no warm-up time, renders negligible problems due to thermal expansion and attendant frequency shifts in the 'buncher cavity, and provides greater efiiciencies and power gains than heretofore realized.

A still further object is to provide an ultra-high frequency amplifier which generates a hollow beam for efficient transfer of energy, minimizes RF cavity losses required to bunch the beam, minimizes debunching of the electron beam itself, does not require a drift space and consequent critical voltage adjustments, and minimizes multipacting in the catcher-collector region.

Still another object is to provide an ultra-high frequency amplifier which is simple to tune and to operate, has a theoretically infinite operational life, and requires no forming-in after a period of non-use.

Other objects will become apparent as the description proceeds.

In the accomplishment of the present invention there is provided an electron discharge device comprising a cavity having two spaced-apart field-defining and electron-emitting surfaces, means for applying an alternating electromagnetic field to the space between saidsurfaces, the spacing between said surfaces and the amplitude and period of said field being such as to produce phase-focusing of electrons in said space, one of said surfaces having an electron-emitting aperture therein from which bunches of phase-focused electrons are emitted, means for accelerating and directing electron bunches periodically emitted from said aperture along a predetermined path, and means for absorbing the kinetic energy from said bunches as they transverse a predetermined region in said path.

The above-mentioned and other features and objects of this invention and the manner of attaining them will become more apparent and the invention itself will be best understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:

FIG. 1 is a longitudinal sectional view of one embodiment of this invention;

FIG. 2 is a fragmentary diagrammatic illustration used in connection with explaining the operation of the buncher-em-itter of the embodiment of FIG. 1;

FIG. 3 is a graph used in connection with explaining the phenomenon of phase focusing;

FIG. 4 is a longitudinal diagrammatic illustration showing the beam path and equipotential surfaces inside the tube of FIG. 1;

FIG. is a similar diagrammatic illustration used in explaining the character of the bunched electron beam;

FIG. 6 is a graph of the wave inside the catcher gap and is used in explaining the theory of operations;

FIG. 7 is a longitudinal sectional illustration of another embodiment of this invention;

FIG. 7a is an enlarged fragmentary longitudinal illustration, in section, of the coaxial line section used in the input and output ends of the tube of both FIGS. 1 and 7;

FIG. 8 is a diagrammatic illustration, in section, of the pulser-emitter of the tube of FIG. 7 and is used in explaining the operation thereof;

FIG. 9 is an enlarged fragmentary illustration, in section, of t he pulse-r-emitter of FIG. 8; and

FIG. is a fragmentary view taken substantially along section line 10-10 of FIG. 7.

Referring to the drawings, and more particularly to FIG. 1, an elongated evacuated cylindrical envelope 1 is hermetically sealed at its opposite ends by means of flexible diaphra-grms or bellows 2 and 3, respectively, wh ch preferably are formed of oxygen-free, high conductivity, copper sheeting. The bellows 2 forms one end wall of an input resonant cavity 4, while the bellows 3 similarly forms one end wall of an output resonant cavity 5. The outer cylindrical wall of these two cavities 4 and 5 are suitably hermetically sealed to the cylindrical wall portion of the envelope 1, such that the cavities form a part of the evacuated envelope.

End bells 6 and 7, respectively, are coaxially threaded onto the respective cavities 4 and 5 as shown. Both of these end bells have differential screw mechanisms mounted therein whereby the cavities 4 and 5 may be finely tuned. While any suitable adjusting mechanism may be used, the one illustrated comprises a sleeve 8 which is coaxially threaded into the tubular extension 9 of the end bell 7 and a section 10 of coaxial line which is threaded into the sleeve 8 as shown, the thread pitch of the external and internal threads of the sleeve 8 being such that rotation of the sleeve 8 will result in vernier or incremental movement of the coaxial line section 10. By way of example, the pitch of the external sleeve threads may be 32-threads-per-inch and the internal threads 24- per-inch. These external threads are arranged such that as the sleeve 8 is screwed into the tubular extension 9, the coaxial section 10 moves outwardly with respect to the sleeve. Thus, for the thread pitches just given, the resultant pitch is 96-threads-per-inch. The screw mechanism 11 at the left-hand end of the tube, as illustrated, is identically constructed to that just described, such that the same reference numerals having the suffix a identify like parts.

The two coaxial line sections 10 and 16a are held against rotation by means of cylindrical, copper electrode supports 12 and 13, respectively, which are hermetically sealed or otherwise secured to the respective bellows 2 and 3. Further, the electrode supports 12 and 13 are conductively rigidly secured to the outer conductors of the respective line sections 10a and 10 as shown. Therefore, turning of the sleeves 8 and 8a will result in axial movement, respectively, of the electrode supports 12 and 13, whereby tuning of the resonant cavities 4 and 5 may be effected. The end walls or bellows 2 and 3 permit this movement without disturbing the vacuum in the tube. Both electrode supports 12 and 13 are provided with angular bores Y14 and 15, respectively, which receive and are insulated from coupling loops 16 and 17, respectively. These loops are connected at one end to the central conductor of the coaxial lines and at the other end to the electrode supports, respectively. The coupling loops 16 and 17 lie within the respective cavities 4 and 5 and serve the purpose of coupling energy thereto or therefrom as the case may be.

Forming another wall of the cavity 4 is a disc 18 of beryllium copper or similar, suitable secondary emitting material having a circular, cup-shaped central portion 19 as shown. This portion 19 is provided with an aperture 20 which is concentric with respect to the longitudinal tube axis 21.

Suitably conductively secured to the electrode support 12 is a dynode 22 of beryllium copper or similar, suitable secondary emitting material having a concave surface 23 which is approximately parallel to the rear surface 24 of the cavity portion 19, but the radius of curvature of surface 23 is slightly smaller than that of surface 24, the reason for this difference becoming apparent from the following description. This surface 23 is preferably partspherical in shape and coaxial with respect to the tube axis 21. Also, these two surfaces 23 and 24, which may be characterized as field-defining surfaces, are spaced apart and form a part of the total resonant cavity 4.

Secured to the right-hand side of the cavity plate 18 is a relatively short, metallic lens sleeve 25. Co-axially sup-. ported within the envelope 1 is a metallic anode or focusing cylinder 26 having its lefthand end disposed adjacent to the sleeve 25. This anode is fabricated, preferably, of a gettering material, such as titanium, for the purpose of scavening residual gases from the tube.

The cavity 5 is constructed quite similarly to that of cavity 4, a disc 27 having a spherically concave portion 28 serving as one end Wall thereof. This concave portion 28 is provided with an aperture 29 which is concentric with respect to the tube axis 21, and it will be noted that this aperture 29 is substantially larger than the aperture 20* in the input cavity 4. The end wall 27 is preferably formed of oxygen-free, high conductivity copper, and desirably may be further treated, such as by gold-plating, in such a manner as to reduce secondary electron emission to a minimum.

Secured to the electrode support 13 is a cylindrical copper collector 30 which is provided with a part-spherical or cup-shaped surface 31 preferably gold-plated. This surface 31 is also coaxial with respect to the tube axis 21 and is spaced from the concave portion 28 as shown to provide a gap 32 for the resonant cavity 5. At this point it may the noted that the size of the aperture 29 is almost as large as the surface 31 itself, such that there is a minimum of overlap between this surface 31 and the concave portion 28 of the cavity wall 27. The reason for this will become apparent from the following description. A short coaxial cylindrical sleeve 33 is secured to the left-hand side of the cavity wall 27 and terminates opposite the right-hand end of the focusing anode sleeve 26 as shown.

A power supply as indicated by the battery 34 is used to provide operating potentials for the tube. The negative terminal is connected to the cavity 4 as shown, while the positive, high-voltage terminal is connected to the cavity 5. An intermediate potential is connected directly to the anode sleeve 26. Typical operating potentials for the illustrated tube may be those shown in the drawing.

In operation, a microwave signal on the order of, for example, lOOO-megacycles is coupled to the coaxial line section 10-2 for exciting the cavity 4. The threaded sleeve 8a may be adjusted for tuning the cavity to resonance, and otherwise the parameters are so selected and adjusted to provide an efiicient impedance match between the line 10a and the cavity 4.

Similarly, a microwave utilization circuit is coupled to the output line 10, and the resonance of the cavity 5 may be tuned by operating the adjusting sleeve 8. The parameters are so selected as to provide a proper impgdance match between the cavity 5 and the line section Just briefly, application of the field in the cavity 4 to the gap between the two surfaces 23 and 24 results in the emission from aperture 28 of recurrent bunches of electrons which are accelerated along the axis 21 and across the gap 32 within the cavity 5. This gap 32 serves as a part of the catcher, such that passage of the electron bunches by the gap 32 results in exciting the cavity 5 and.

the consequent generation of an amplified microwave signal. This microwave signal is coupled from the cavity 5 by means of the loop 17 and the line extension 10. As will 'be noted, the back side of the cavity 32 is terminated 'by means of the total surface 31, such that this suurface serves as the collector of the electron bunches.

Since there are several unique features in the various parts and functions just explained, they will be individually and then collectively treated in the following. The first unique feature to be considered is the pulser-emitter or swing of the applied electric field. This bunching process is herein characterized as phase-focusing.

m'ulti-pactor which is composed of the two field-defining,

electron-emitting surfaces 23 and 24 within the cavity 4. This particular assembly performs essentially two different functions simultaneously, these being the emission of electrons and the bunching or phase-focusing of these electrons into periodic or recurrent groups which are emitted from the aperture 20 in spaced relation. The two surfaces 23 and 24 may be considered as constituting the dynodes of a multipactor such that electrons oscillating with impacting velocities between these two surfaces produce secondary electrons. Since the secondaries also oscillate with impacting velocities, they in turn produce secondaries such that the number of electrons multiplies until certain limiting conditions are reached. Under certain conditions, after making a number of trips between the surfaces, the electrons will become bunched, or in other words, phase-focused. This may be better explained as follows.

Referring to FIGS. 2 and 3, where like numerals indicate like parts, it is first assumed that an alternating electromagnetic field is applied to the gap between the surfaces 23 and 24. This alternating field is illustrated by the sine waves in FIG. 3. It is also assumed that a single electron departs, by reason'of photoemission, thermal emission or other electron-emitting disturbance, from point 35 in FIG. 2 and is drawn by the field toward the surface 24 along the path indicated by the arrow 35a. If the electron arrives at the surface 24 with sufficient kinetic energy, secondary electrons will be produced. There are, however, two possibilities to be considered for the secondary electrons; one where at the instant of mere emergence, the initial energy may be insufiicient to overcome the field which may still have the same direction which forced the primary electron to move toward surface 24. Under this condition, these secondary electrons will be pushed back and will be lost for any further action.

However, considering the second possibility, namely where the electric field changes direction while the primary electron is approaching the surface 24, the secondary electrons will experience favorable conditions for departure and travel back toward surface 23. The arrow 36 indicates this return travel. If, upon arrival of these secondaries at the surface 23, similar conditions should prevail as they existed when they departed from surface 24, additional secondary electrons are produced and the process continues with oscillatory traversals producing an electron-population growth.

In order for this population growth to develop, it is necessary for the electron transit time from one electrode to another to take precisely odd numbers of half the period of the applied alternating electromagnetic field. Since the transit time depends upon the applied voltage, this condition can be fulfilled by adjusting the amplitude and/or frequency of the driving field. While at first blush it would appear that this electron multiplication would continue ad infinitum, due to space charge buildup within the gap, the multiplication reaches, eventually, a limiting value as is explained in the following.

The electrons which depart at different instances from one dynode, e.-g., dynode 23, tend to catch up with each other, so that upon arrival at the opposite electrode they show an improvement in their synchronization. Repeated trips of these electrons result in a tightly bunched electron-sheet, steadily increasing in its density and bouncing back and forth between the electrodes and following the This phase-focusing is illustrated in FIG. 3 wherein the two sine waves illustrate the field which is swinging between the two surfaces 23 and 24. If it is first assumed that an electron a departs from the surface 24 at time t it will be accelerated across the gap and will impact the surface 23, as indicate dby the symbol a at time t the time interval between t and t being equal .to a half period of sine Wave A, B. Next assume that another electron b departs from surface 24 at time 1 This electron starts sometime after the zero potential point of the field and experiences greater acceleration initiall than did the preceding electron a. This electron b there-upon arrives at the surface 23 at time t which corresponds to slightly less than a half period of the sine wave. Still further, an electron c leaving surface 24 at time t will arrive at surface 23 at time t the time interval still being shorter than b and less than a half period of the sine wave. As graphically shown by the spacing between the points a, b and c and the spacing between points a, b and c, it will at once be recognized that the electrons upon arriving at surface 23 are closer together than when they departed from the surface 24.

Continuing with the cycling, the respective electrons denoted by the letters a, b and c eventually arrive at spaced intervals a", b" and c" on the surface 24 and are still more closely spaced than they were at surface 23, such that it at once becomes apparent that after repetitive cycling, all of the electrons will arrive at the two surfaces 23 and 24 approaching synchronism as a limit. The electrons may thus be considered as being tightly bunched or phase-focused.

While it may be assumed that all electrons emitted from the two surfaces 23 and 24 will approach phase sync-hronism, this is not necessarily so. Referring particularly to FIG. 3 of the drawings, analysis reveals that only about 35 of the field period (this 35 including the interval between .t and I is efiective in bunching the electrons. In other words, electrons emitted from one surface 23, 24 in phase with this 35 portion will be eventually bunched as previously explained. However, electrons emitted at other times during the period will experience a retarding rather than an accelerating field and thus give up the energy thereof to the field.

Now referring to FIG. 2, if it is assumed that the electron multiplication and phasing occurs between the surfaces 23 and 24 in directions corresponding to the arrows, when the phased electrons reach the vicinity of aperture 2%, they will be emitted cyclically in somewhat ring-form with the heaviest electron density being in the peripheral portions of the rings. The emission thereupon takes the form of the hollow electron rings 37 diagrammatically shown in FIG. 2. The spacing between these rings corresponds to the frequency of the exciting field, with the electrons in each of the rings 37 tightly bunched and dense.

By reason of the various supply voltages applied to the tube parts as previously explained, equipotential surfaces are setup within the tube something on the order of that shown in FIG. 4. A convergent electron lens is provided between the nurface 24 and the focusing anode 26 Which tends to focus or converge the successive, longitudinally aligned electron bunches 37, hereinafter referred to as a beam 38, onto the axis 21 (FIGS. 4, 5). However, as the beam progresses through the tube, it experiences the action of the divergent lens between the focusmg anode 26 and the catcher 28, 31 and eventually passes through the aperture 29 and across the gap 32 to be collected by the surface 31.

Also, as a part of this beam-focusing consideration, it will be noted that the dynode is symmetrically curved with respect to the tube axis 21. This is important for the reason that secondary electrons leave the surface 23 with an approximate Maxwellian velocity distribution having a mean energy of about ten (10) electron volts, and directions corresponding to a Lambert distribution. Both the velocity and directional distributions are remarkably independent of the energy and angle of incidence of the primary electron. The secondary electrons finally passing through the aperture thus depart from the surface 23 at an angle substantially perpendicular thereto. Since this surface 23 is angled toward the axis 21, initial electron movement is convergent.

Thus, two converging forces are brought to bear on the emitted electrons, one due to the focusing anode 26 as previously explained, and a second due to the velocity and directional distribution of the electrons upon departure from the dynode surface 23. This convergence is of valued importance for the reason that it results in tighter packing of the electrons in the bunches 37 and reduces radiation losses.

Referring once again to FIG. 2, the multiplied electrons migrate radially inwardly toward the aperture 20 for several reasons. Firstly, they experience the attractive force of the field provided by the focusing anode 26 and are therefore drawn through the aperture 20. Secondly, space charge repulsion of the oscillating electron cloud in the 'gap 23a tends to move the bunches inwardly. Thirdly, the electrode 23 has less curvature than surface 24, as mentioned previously. By reason of this, the field forces cause the electrons to move toward the tube axis. The conjoint action of these three forces results in the emission of the bunches 37.

A second limiting factor with respect to the ultimate number of emitted electrons is due to space charge effects. For a sustained multipacting process, the secondary electron yield of the surfaces 23 and 24 must be greater than unity. The electron cloud thus builds up by the multiplication process of impinging electrons twice every field cycle. This, therefore, suggests an increase in the number of emerging electrons without limit; however, an equilibrium state is approached when the emerging electrons near the boundary surfaces 23, 24 experience a large space charge field which prevents part of them from joining the multipacting electron cloud and finally impinging the opposite surface. Thus, when this space charge limit is reached, there will be only one emerging electron for every impinging electron which can overcome the space charge field. The additional secondary electrons are driven back into the emitting surface.

As will now appear, there are simultaneously three processes in operation within the gap 23a defined by the two multipacting surfaces 23 and 24. These are (1) electron multiplication at the boundary surfaces, (2) energy transfer from the field to the moving space charge, and (3) the space charge rocesses which limit the ultimate electron number density. These processes counterbalance each other to provide a stable, self-sustained operation which may be defined as the state of dynamic equilibrium.

Multiplicating between the two surfaces 23 and 24 may be considered as substantially a constant voltage process. The voltage necessary for producing multiplicating is such that the electron transit time corresponds to onehalf period of the applied frequency.

It should be noted that two different modes of multipactor operation are possible. A multipactor may, first, be designed and operated such that the points of impact of the electrons on the first, third, fifth, etc. trips are well separated: in this event there will exist only a few electrons at the input end 23b of the gap 230, with the number thereof increasing toward the output end 20. If a few more electrons are injected at the input end, the output electrons will increase in number. The multipactor thus operates as an electron multiplier. In order for the electron output to be a faithful reproduction of the electron input, the space charge should be low even at the output end 20.

This is not, however, the mode in which the multipactor is operated in this invention. In this case, it must be considered that the secondary electrons at a given point-of emission may have any direction, even though the majority leave perpendicular to the surface. Thus, after a few trips, some secondaries travel backwardly, i.e., in a direction toward the input end 23b, so that the different stages of multiplication are not discretely separated in space. If, in addition, the multipactor is operated at space charge saturation, space charge will be appreciable even at the input end 23b, and additional electrons which happen to enter it will have almost no effect. Even though the electron output thus is not representative of the number of electrons entering the input end 2317, it nevertheless still depends, as will be explained, on the amplitude of the RF input voltage. The device is thus a voltage amplifier, and this is the desired mode of operation.

Now, some consideration should be given to the noise to which this amplifier is subject. There are several sources of noise:

(a) Electrons randomly arriving in time at the input end 23b can seriously affect operation in the multiplier mode, but are not noticeable in the amplifier mode;

(b) Noise attributable to fluctuations corresponding to the electron temperature of the source (the input cavity 4) will be reduced by the space charge in the amplifier much as it is in an ordinary thermionic tube;

(0) Noise produced by ions entering the input end 23b or generated within the gap 23a can be appreciable; however, these migrate towards the heaviest electron concentration at the output end 20 and are scavenged by the electron bunches 37, thereby minimizing noise attributable thereto.

Thus, the amplifier is relatively noise-free.

Further features of the invention become clear when the amplifier is considered as a circuit element. As FIG. 4 shows, the equi-potential surfaces are somewhat curved at the multipactor aperture. Thus, the field of anode 26 aids in extracting the electron pulses 37 from the aperture 20. As a matter of fact, a desirable design is one in which the gradient at the multipactor exit surface (central portion of surface 23) exceeds that within the gap 23a. In this case, although the current is space charge limited within the gap 23a, the electrons leaving the aperture 20 are not limited by space charge and can freely emerge.

Now, while the cavity 4 has a high Q as long as it is unloaded, Q is reduced as the space charge builds up in the multipactor gap 23a, and even more so as electrons are emitted from aperture '20 into the focusing anode 26. The result is the same as if an equivalent resistor were connected between the surfaces 23 and 24 adjacent to aperture 20, the ohmic value of which decreases as the number of electrons in the multipactor becomes larger.

Thus, as the voltage delivered by the input coupling loop 16 increases, this equivalent resistor becomes smaller and the voltage across the gap 23a increases less than in proportion to the increase in input voltage and within this range is still responsive to RF amplitude. In a manner of speaking, the variable output load has a servo effect which linearizes the amplifier.

Two additional observations may be made. Firstly, depending on the power range in which the tube is to be operated (for example, between 1 to 50 or 10 to 500 watts), the average Q of the cavity and therefore the correct size of the coupling loop 16 will change. As is well known, this loop serves to match the cavity impedance to the characteristic impedance of the coaxial line input; hence, this loop must be designed for the desired power range.

Secondly, since the field of the anode 26 (as seen in FIG. 4) penetrates somewhat into the multipactor gap 23a, some energy is supplied from the anode 26. This is explained more fully later on in connection with the preferred embodiment as shown in FIG. 7.

At this point, it will be recognized that while the working embodiment illustrated in FIG. 1 of the drawings utilizes a resonant cavity 4 for applying the field to the multipacting gap, still other conventional arrangements are indeed possible. In the illustrated embodiment, an inductive loop provides the coupling, but instead, within the skill of the art, a capacity probe, a coaxial transmission line transformer, a waveguide input, and other means may be used for providing this coupling.

As explained earlier, a beam 38 of electron bunches 37 is emitted from the multipactor aperture 20. This beam passes through the tube to the catcher 28, 31 along a pencil-like path free of structure and shaped something along the order of that indicated by the numeral 38 in FIG. 5, the converging and diverging lenses previously described being partially responsible for this shape.

The pencil-like path of the beam will be alfected by space charge action between successive electron bunches 37 as previously explained; however, it may be modified by positive ions which are always present in a tube, even at the operating pressure of 1() to 10- millimeters of mercury. These ions in the main space of the tube are, from a noise standpoint, less harmful than the ones in the multipactor gap 23a. They will tend to congregate in the space charge bunches 37 or to oscillate around them, predominantly in radial directions. It might be thought that they would eventually be removed by recombination; however, this is a less probable event than that of charge interchange between ions and neutral molecules. In the latter case, a new ion is formed, which, however, has less energy, while the previous ion, which is now neutral, moves radially away, hitting the focusing anode 26 or the tube walls. It is, therefore, preferred to fabricate the focusing anode 26 of titanium or the like or to cover it with a suitable getter so as to absorb residual gasses.

As previously explained, the beam 38 is eventually collected by the electrode 31 after passing the gap 32. The lens optics are so designed that the beam enters the gap 32 as closely as possible to the edges of the opening 29. Since the beam 38 is composed primarily of electron bunches in annular form, this means that the heaviest concentration of electrons will be immediately adjacent to the gap 32 which may be excited.

As is well known in connection with any catcher, passage of the electron-bunches therethrough results in the absorption of the electron energy and the generation of an electromagnetic field in response thereto. As in the case of the input end of the tube, the impedance of the gap 32 must be matched to the output line section 10 before maximum efficiency in operation may be obtained. Also, the cavity must resonate at the period of the bunched electrons passing the gap 32.

Since the electron bunches are accelerated upon passing through the tube from the pulser-emitter to the collector, they will acquire a substantial amount of kinetic energy. This kinetic energy is absorbed in the gap 32 in the development of the resonant field in the cavity 5, and this developed field may be considered to be something along the order of that shown in FIG. 6. Of importance is the fact that the electron bunches upon entering the gap 32 experience the retarding force of the generated field such that by the time the bunches arrive at the surface 31, the velocities thereof have been materially reduced to a low value. The increment of velocity remaining is just enough for the surface 31 to collect the electrons. Thus, even though the kinetic energy of the electron bunches just passing the gap 32 may be high, arrival of these bunches at the surface 31 will not produce secondary emission because of the substantial absorption of this energy and reduction of electron velocity. Additionally, the catcher DC. voltage is adjusted so that with a given gap width multipacting cannot occur. This explanation is, of course, idealized, since there are certain limiting conditions which,

16 if exceeded, could produce a modicum of secondary emission.

One method of controlling electron collection on the surface 31 is the provision of a suitable resistor 40 in series with the high voltage supply terminal of the battery 34 (FIG. 5). Referring to FIG. 5, and assuming that the tube is operating, collection of electrons by the surface 31 will produce a current flowing in the direction of the arrow 41 which, by reason of the voltage drop across resistor 46, lowers the voltage on the surface 31. This lowered voltage will correspond to the shaded increment 42 in the negative lobe of the voltage wave shown in FIG. 6. Since the electrons entering the gap 32 now encounter the increased negative field produced by this more negative increment 42, they will be slowed down more than previously such that they will impact the surface 31 with less velocity than before. This more negative buildup on the surface 31 will increase until such time as the collection of electrons by the surface 31 and the current indicated by the arrow 41 reach equilibrium values which are thereafter self-controlling.

Referring to FIGS. 7, 8, 9 and 10, a second, preferred embodiment of this invention will now be described. Since. much of the structure in this second embodiment is a duplication of that of FIG. 1, like numerals are used to indicate like parts.

The primary differences in the two structures reside in the pulser-emitter, the focusing anode and the catcher. With respect to the pulser-emitter, which in FIGIl comprises the axially spaced surfaces 23 and 24, the pulseremitter of FIG. 7 comprises two coaxial, radially spaced, annular surfaces. This latter pulser-emitter includes a dynode element 37 of cylindrical shape which is fixedly and conductively secured to the electrode support 12. This dynode element 37 is coaxial with respect to the tube axis 21 and terminates in a fiat face 38 normal to the axis 21. A portion of the outer periphery of the element 37 immediately adjacent the face 38 is provided with an annular recess or surface which, in cross-section, is curved as shown.

The end plate 18 of the cavity 4 has an annular hub portion 42 which is provided on its inner periphery with an annular surface 43 which, in cross-section, is curved as shown. The two annular surfaces 39 and 43 are coaxial with respect to the tube axis 21 and are concave facing each other. They are also radially spaced apart.

The surface 43 is provided with a surface extension 44 which diverges as shown. Preferably, this surface extension 44 is frusto-conical in shape, and is machined at an angle of approximately 15 with respect to the axis 21.

Instead of the cylindrical anode 26 as shown in FIG. 1, this embodiment employs two axially separated and insulated anode sleeves 26a and 26b. The opposite ends of these sleeves taper inwardly as shown. Three ceramic or the like rods 26c spaced apart are fixedly secured to and support these sleeves 26a, 26b, and are secured to the end plate 27 by some suitable means such as rivets 27a passing through the sleeve extension 33.

In operation, the surfaces 39 and 43 define the multipacting gap and therefore are formed of a suitable secondary emissive material which may be the same as that used in the embodiment of FIG. 1 for the surface 23 and '24. An exciting field is applied to the cavity 4 the same as previously explained in connection with FIG. 1, the differential screw mechanism 11 being adjusted until proper resonance is obtained. An alternating field is established across the multipacting gap which results in the multiplication and phase focusing of electrons which are emitted by the dynode surfaces 39 and 43. By reason of the positive potential field established by the focusing anodes 26a, 26b the electrons generated within the multipacting gap are caused to migrate in a direction something along the order indicated by the arrows 45 in FIG. 8 until they finally emerge from the annular opening 46 of the gap as a converging beam 38a which is composed of annular bunches 37a of electrons. The anodes 26a, 26b in cooperation with the pulser-emitter form an electron lens as in the case of the embodiment of FIG. 1 which initially converges the beam 38a toward the axis 21. However, anodes 26a and 26b have applied thereto different adjustable accelerating voltages as shown whereby adjustment of the focusing may be effected. Also these anodes may have applied thereto suitable signals for modulating the beam 38a if this should be desired.

The dynode element 37 is axially adjusted such that the final impact of the oscillating electrons between the dynode surfaces 39 and 43 will be against the surface extension 44. This is important for the reason that, as previously explained, the secondary electrons which leave the surface extension 44 depart with an approximate Maxwellian velocity distribution and a Lambert directional distribution. This velocity distribution has a mean energy of about electron volts and the directional distribution is substantially perpendicular to the surface 44. Thus, the finally departing electrons, which, as in the embodiment of FIG. 1, are phase-focused, depart from the surface 44 in a direction corresponding to the arrow 47 such that the initial electron emission is in a direction toward the axis 21, or in other words is convergent. Also the electrons upon leaving the surface 44 are still under the influence of the fringe RF field of the gap which accelerates them toward the axis 21. The combination of these effects results in the formation of the converging beam 33a which produces the favorable results of more tightly packed bunches 37a and also reduces radiation losses by keeping the beam away from the wall.

The preceding discussions regarding the generation of the electrons may now be generally considered with respect to the bunching thereof. There are three different ways to form dense bunches:

(1) Velocity modulation of the type well known for the klystron;

(2) Phase-focusing as described herein as an iterated form of velocity modulation; and

(3) Secondary emission whereby additional electrons are continuously being generated.

The latter two of these processes are performed by the multipacting pulser-emitter of this invention with all electrons within the gap (FIG. 7) ultimately being bunched.

At this point in this discussion, it is well to recognize and compare the operating features of the pulser-emitters of the two embodiments of FIGS. 1 and 7. In both cases the opposed surfaces (23 and 24 in FIG. 1 and 39 and 43 in FIG. 7) are at the same DC. potential but have an RF potential difference therebetween. This gives rise to two different modes of multipacting operation as follows. Thefirst mode is characterized by the absence of a DC. anodic field between the two surfaces while in the second mode by the presence thereof.

In the first mode (referring to FIG. 1) wherein little or no anodic, D.C. field is present in the gap between the two surfaces 23 and 24, electron multipaction requires the following:

(a) Electrons leaving one surface must be accelerated by the RF field to strike the opposing surface with enough energy to eject secondaries at a rate greater than unity; and

(b) When electrons strike the second surface, the RF field must not have a direction or intensity which will prevent the emission of the secondary electrons.

In the second mode (referring now to FIG. 7) wherein, as will be explained later, an anodic field is present between the surfaces 39 and 43, electron multipaction requires the same RF field conditions of paragraph (a), but is more or less independent of those of paragraph (b); however, an improvement in operating efficiency is realized. As explained previously, those electrons emitted within a certain phase range are phase-focused, and those emitted outside this angle are turned back into the emitting surface and lost. However, by introducing the aforementioned anodic field, these latter electrons are not turned back by the RF field and lost, but instead are joined with those which are phase-focused. The anodic field insures emission of electrons at all times, if sufiiciently strong, even against any retarding forces of the RF field. This is explained as follows: the positive field produced by the focusing anode 26a penetrates the gap between the surfaces 39 and 43 something along the order of that diagrammed in FIG. 9. This field inside the gap at about the center thereof appears to the electrons as an anode such that upon emission they are accelerated toward the opposite surface. If such electrons are emitted at a time which falls outside the aforementioned phase range, instead of being accelerated by the RF field, they will be retarded. They fall short of the opposite surface and make a return trip under the influence of the anode field toward the first surface, during which they continue to be decelerated by the RF field. This oscillatory action continues short of impacting the surfaces 39, 43; however, the electrons migrate toward the opening 46. This condition is illustrated by the dashed serpentine arrow in FIG. 9. However, some of these electrons may be retarded sufficiently before emerging from opening 46 as to fall into step, or in other words into the focusing phase range, with the electrons which are being phase-focused: this is illustrated by the solid line arrow 45a in FIG. 9. As will now be apparent, those electrons which encounter retarding rather than accelerating forces are eventually oscillated, or in other words quasi-phase focused, into a phase relationship with the RF field in which they join those electrons which are phase-focused. Thus, substan tially all of the electrons emitted by the surfaces 39, 43 eventually emerge from the opening 46 in the form of the bunches 37a, and the electrons which are initially emitted outside of the focusing phase range are not lost.

When a central anodic field is provided so that emission of electrons can occur against the RF field, work is performed by the electrons against the RF field and the energy of the latter is thereby increased; This serves in reinforcing the RF field or otherwise restoring the energy absorbed from this field by the process of phasefocusing, thereby reducing the amount of power required to drive the multipactor. The multipactor is, therefore, an efiicient current generator. While use of the central anodic field affects electron transit time, this effect is readily compensated by changing the spacing of the multipacting surfaces and/ or the cavity tuning.

The curvatures of the dynode surfaces 39 and 43 are related to a certain extent to the voltages applied to the focusing anodes 26a, 2617. This relationship is such that the curvatures may be increased as a consequence of increasing anode voltage in order to maintain space charge saturation. The opposite condition is also true.

While the surfaces 39 and 43 are illustrated and have been described in the preceding as being curved in crosssection, it will appear as obvious to persons skilled in the art that they may be flat or of some other shape and still be operative within the spirit and scope of this invention. Actually, the shapes of surfaces 39 and 43 will depend to a large extent upon design preferences and requirements in a final tube structure.

With reference to the catcher of FIG. 7, it is quite similar to that of FIG. 1 but differs primarily in the construction of the collector and apertured cavity end plate. Collector 30a is cup-shaped and is provided with a curved surface 31a like that of surface 31 in FIG. 1. However, a coaxial cylindrical recess 43 is provided in collector 30a which extends into the supporting member 13a. Over the open end of recess 48 is an open mesh or conductive screen 49 which is suitably conductively secured to collector 30a. End plate 27 is fiat as shown and is provided with a coaxial aperture 29a of smaller diameter than aper- 13 ture 29 of FIG. 1. Actually this aperture 29a is of about the same diameter as recess 48 and in an operating embodiment is about one-half /2) inch, the dimensions of the remaining parts of the tube being as stated hereinafter.

The operation of this catcher is essentially the same as that of FIG. 1, except the beam 38a is of a size which just clears the aperture 29a and the opening of the recess 48. The cavity 5 is excited by the beam bunches 37a as in the case of FIG. 1 as they pass the gap between the plate 27 and the surface provided by the mesh 49 and surface 31a. Since secondary emission from the collector is to be avoided or minimized, this surface 31a as well as the mesh 49 are preferably gold plated. The electrons penetrating the mesh 49 strike the wall of the recess 48 and are there, at least partially, collected. If any secondary electrons are emitted from the wall, they will be trapped for the most part within the recess and will travel radially thereacross. As space charge builds up, such electrons will be repelled to the wall and be thereby collected. In order to reduce secondary emission, the wall is preferably gold plated. As will now be apparent, deleterious effects of secondary emission within the catcher are either avoided or materially minimized.

From the preceding explanations, it will be evident that tube operation (with respect to both embodiments) may be almost instantaneously started inasmuch as there is no thermionic cathode to be heated. Further than this, by use of the multipacting process, the bunches of electrons emitted from the multipactor aperture or annulus are tightly packed. This leads to efficiency in operation inasmuch as there are fewer electrons randomly spaced in between the bunches. Additionally, the longitudinal distance inside the tube between the pulser-emitter and the catcher may be used for purposes of accelerating the electron bunches and thereby imparting greater kinetic energy thereto such that by the time the bunches reach the catcher, the energy available for absorption is substantial thereby resulting in the development of a high level of power output. Further than this, since secondary emission at these high kinetic energies from the surface 31 is maintained at a low level, and the velocities of the electrons which reach the surface 31 are relatively low, there is very little energy absorbed by the surface 31 which can be transformed into heat. Thus, the surface 31, hence the collector 30, operates at a fairly low temperature and does not require the use of extraordinary cooling devices. Since the tube is not limited by thermionic current in its operation, it is not limited as to power which can be developed except as to its capability of dissipating heat which is internally generated. In this connection, the scheme of the resistor 40 in FIG. 5 is important inasmuch as it is located externally of the tube and therefore in reality removes a portion of the heat from the tube which normally would be developed at the surface 31.

As will appear obvious from the preceding description, the size, shape, dimensions and operating parameters may be changed and adjusted without departing from the spirit and scope of this invention. Some dimensions are given in the following of working embodiments of this invention and it is to be understood that these dimensions are given by way of example only and are not to be considered as limitations.

Spacing between surfaces 23 and 24 inohes 0.220

Aperture 29a do /2 Recess 48 do /8 Recess 48 inches deep Widest space between surfaces 39 and 43 inches A Space between surfaces 39 and 43 at edges do Axial distance from surface 38b (FIG. 7)

to end plate 77 do 1 FIGS. 1 and 7 are otherwise drawn to scale such that if other dimensions are desired, they can be taken directly from the drawing.

While I have described above the principles of my in vention in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention.

What is claimed is:

1. An electron discharge device comprising cathode means for emitting spaced bunches of electrons in a given direction, means for accelerating said bunches in beamlike form in said direction along a predetermined pencillike path, and catcher means for absorbing energy from all of said bunches after one traversal of said path, and for generating an electrical signal in response thereto, said path being free of structure and extending from said cathode means to said catcher means, said bunches thereby traversing said path without encountering any structure therein.

2. An electron discharge device comprising multipacting means for emitting periodic bunches of electrons in a given direction, means for accelerating said bunches along a predetermined pencil-like path, means including said multipacting and accelerating means for converging said bunches along said path, and catcher means intercepting all of said bunches after one traversal of said path for generating an electrical signal in response thereto, said path being free of structure and extending from said cathode means to said catcher means. I

3. An electron discharge device according to claim 1, in which said cathode means includes multipacting means for emitting annular bunches of electrons.

4. An electron discharge device comprising two separated electrodes having opposed secondary emissive surfaces, said surfaces having a non-uniform spacing the-rebetween whereby they are more widely separated in one region than in an adjacent region, means for applying an alternating electric field between said surfaces for phasefocusing secondary electrons emitted from said surfaces to form a concentration of secondary electrons in the region of widest separation of said surfaces, and means including one of said electrodes providing an opening through which said concentration of secondary electrons are projected.

5. An electron discharge device comprising two separated electrodes having opposed secondary emissive surfaces, said surfaces having curved contours of different radii being more widely separated in one region, means for producing an alternating field between said surfaces for phase-focusing secondary electrons emitted from said surfaces to form a concentration of secondary electrons in said one region, and one of said electrodes having an opening through which said concentration of secondary electrons are projected.

6. An electron discharge device comprising two separated juxtaposed electrodes having opposed secondary electron emissive surfaces; means for varying the separation of said surfaces; means for producing an alternating electric field between said surfaces; said surfaces having predetermined contours for phase-focusing secondary electrons emitted from said surfaces by said alternatnig field to form periodic bunches of secondary electrons; and means providing for the emission of electron bunches from between said surfaces.

7. An electron discharge device according to claim 6, in which said field-producing means comprises a resonat- 15 ing circuit of which said surfaces constitute the major portion of the circuit capacitance parameter.

8. An electron discharge device according to claim 6, in which said field-producing means comprises a cavity resonator of which said surfaces constitute the major portion of the resonator capacitance parameter.

9. An electron discharge device according to claim 6, in which said field-producing means comprises a cavity resonator of which said surfaces constitute the major portion of the resonator capacitance parameter, means for accelerating said electron bunches to increase the kinetic energy thereof; and catcher means for converting said kinetic energy into an alternating current.

10. An electron discharge device comprising two spaced apart field-defining surfaces, means for applying an alternating electro-magnetic field to the space between said surfaces, the spacing between said surfaces and the amplitude and period of said field being such as to produce phase-focusing of electrons in said space, one of said surfaces having an electron-emitting aperture therein from which bunches of phase-focused electrons are emitted, means for accelerating and directing electron bunches periodically emitted from said aperture along a predetermined path, and means for absorbing the kinetic energy from said bunches as they traverse a predetermined region in said path.

11. An electron discharge device comprising two spaced apart field-defining surfaces, means for applying an alternating electro-magnetic field to the space between said surfaces, the spacing between said surfaces and the amplitude and period of said field being such as to produce phase-focusing of electrons in said space, one of said surfaces having an electron-emitting aperture therein from Which bunches of phase-focused electrons are emitted, the size of said aperture being such that said bunches are emitted in ring-like form, means for accelerating and directing electron bunches periodically emitted from said aperture along a predetermined path, and means for absorbing the kinetic energy from said bunches as they traverse a predetermined region in said path.

12. An electron discharge device comprising two spaced apart field-defining surfaces, means for applying an alternating electromagnetic field to the space between said surfaces, the spacing between said surfaces and the amplitude and period of said field being such as to produce phase-focusing of electrons in said space, one of said surfaces having an electron-emitting aperture therein from which bunches of phase-focused electrons are emitted, the size of said aperture being such that said bunches are emitted in annular form, means for directing electrons in said space toward said aperture and for accelerating electron bunches periodically emitted from said aperture along a predetermined path, and catcher means utilizing said accelerated bunches for generating a signal in response thereto.

13. An electron discharge device comprising a cavity having two spaced apart field-defining surfaces, means for applying an alternating electromagnetic field to the space between said surfaces, at least one of said surfaces being of a material having a secondary emission ratio greater than unity, the spacing between said surfaces and the amplitude and spacing of said field being such that electrons oscillate between surfaces and are phasefocused by said field, one of said surfaces having an electron-emitting aperture therein from which bunches of phase-focused electrons are emitted, means for accelerating and directing electron bunches periodically emitted from said aperture along a predetermined path, and means for absorbing the kinetic energy from said bunches as they traverse a predetermined region in said path.

14. The device of claim 13 wherein said field-defining surfaces are facing each other.

15. An electron discharge device comprising a cavity having two spaced apart field-defining surfaces, means for applying an alternating electromagnetic field to the space between said surfaces, at least one of said surfaces being of a material having a secondary emission ratio greater than unity, the spacing between said surfaces and the amplitude and period of said field being such as to oscillate electrons between said surfaces and to produce phase-focusing of electrons in said space, one of said surfaces having an electron-emittin g aperture therein from which bunches of phase-focused electrons are emitted, the size of said aperture being such that said bunches are emitted in annular form, means for directing electrons in said space toward said aperture and for accelerating electron bunches periodically emitted from said aperture along a predetermined path, said predetermined path being substantially symmetrical about an axis, and catcher means utilizing said accelerated bunches for generating a signal in response thereto.

16. An electron discharge device comprising a cavity having tWo spaced apart field-defining surfaces, said surfaces being secondary emissive, means for applying an alternating electromagnetic field to the space between said surfaces, the spacing between said surfaces and the amplitude and period of said field being such as to produce phase-focusing of electrons in said space, one of said surfaces having a centrally located electron-emitting aperture from which periodic bunches of phase-focused electrons are emitted, electron-optical means for directing electrons in said space toward said aperture and for forming said bunches into a beam which converges into a focal region spaced from said aperture, and catcher means utilizing said divergent beam for generating a signal in response to said periodic bunches of electrons.

17. An electron discharge device comprising a cavity having two spaced apart and opposed field-defining surfaces, said surfaces being secondary emissive, means for applying an alternating electromagnetic field to the space between said surfaces, the spacing between said surfaces and the amplitude and period of said field being such as to produce phase-focusing of electrons in said space, one of said surfaces having a centrally located electronemitting aperture from which periodic bunches of phasefocused electrons are emitted, a cylindrical accelerating anode having an axis which is normal to the plane of said aperture and also passes through the center thereof, one end of said anode being located adjacent to said apertured surface, both said surfaces being concave facing said one end for providing a convergent electron lens therewith; a catcher cavity having two electrodes spaced apart to provide a gap therebetween, one of said electrodes having a surface disposed opposite the other end of said anode and intersected by said axis, the other electrode having a beam-receiving aperture concentric with said axis and being interposed between said one electrode and said other anode end.

18. An electron discharge device comprising a cavity having two spaced apart facing field-defining surfaces, said surfaces being secondary emissive, means for applying an alternating electromagnetic field to the space between said surfaces, the spacing between said surfaces and the amplitude and period of said field being such as to produce phase-focusing of electrons in said space, one of said surfaces having a centrally located electron-emitting aperture from which periodic bunches of phasefocused electrons are emitted, a cylindrical anode having an axis which is normal to the plane of said aperture and also passes through the center thereof, one end of said anode being located adjacent to said apertured surface, both said surfaces being concave facing said one end for providing a convergent electron lens therewith; a catcher cavity having two electrodes spaced apart to provide a gap therebetween; one of said electrodes having a concave surface dispose-d opposite the other end of said anode and intersected at right angles by said axis, said one electrode surface being of a material having a secondary emission ratio of less than unity, the other electrode having a beam-receiving aperture concentric 17 with said axis and being interposed between said one electrode and said other anode end.-

19. An electron discharge device comprising an evacuated envelope having opposite spaced apart ends, a resonant input cavity in one of said ends, an input circuit coupled to said cavity for applying a resonating signal thereto, said cavity having two spaced apart and substantially parallel field-defining surfaces, the gap between said surfaces being resonant at a predetermined frequency, said surfaces having a secondary emission ratio of greater than unity, one of said surfaces having a centrally located electron-emitting aperture from which periodic bunches of phase-focused electrons may be emitted, a cylindrical anode having an axis which is normal to the plane of said aperture and also passes through the center thereof, one end of said anode being located adjacent to said apertured surface, both said surfaces being concave facing said one end for providing a convergent electron lens therewith; a resonant output cavity in the other end of said envelope, said output cavity having two electrodes spaced apart to provide a gap therebetween, one of said electrodes having a concave surface disposed opposite the other end of said anode and intersected at right angles by said axis, said one electrode surface being of a material having a secondary emission ratio of less than unity, the other electrode having a beam-receiving aperture concentric with said axis and being interposed between said one electrode and said other anode end, said output gap being resonant at a predetermined frequency and gene-rating an alternating signal at the same frequency in response to the interception of periodic bunches of electrons.

20. The device of claim 19 including means for selectively adjusting the resonant frequency of said cavities.

21. An electron discharge device comprising two electrodes having coaxial annular radially spaced and radially opposed secondary emissive surfaces, and means for applying an alternating field between said surfaces for phase-focusing electrons therebetween, said surfaces defining an annular opening through which bunches of phase-focused electrons may be emitted.

22. An electron discharge device comprising two electrodes having coaxial annular radially spaced and radially opposed secondary emissive surfaces, means for applying an alternating field between said surfaces for phase-focusing electrons therebetween, said surfaces defining an annular opening through which bunches of phase-focused electrons may be emitted, and means for directing electrons from said opening in a direction toward the axis of said surfaces.

23. An electron discharge device comprising two electrodes having coaxial annular radially spaced radially orpposed secondary emissive surfaces, means for applying an alternating field between said surfaces for phase-focusing electrons therebetween, said surfaces defining an annular opening through which bunches of phase-focused electrons may be emitted, means for accelerating and converging electrons emitted from said opening along and toward the axis of said surfaces, and catcher means for intercepting said accelerated electrons for generating a signal in response thereto.

24. An electron discharge device comprising two electrodes having coaxial annular radially spaced secondary emissive surfaces, the outer of said surfaces having an annular surface extension which diverges outwardly, and means for applying an alternating field between said surfaces for phase-focusing electrons therebetween, said surfaces defining an annular opening through which bunches of phase-focusing electrons may be emitted.

25. An electron discharge device comprising two electrodes having coaxial annular radially spaced secondary emissive surfaces, the outer of said surfaces having an annular surface extension which diverges outwardly, means for applying an alternating field between said surfaces for phase-focusing electrons therebetween, said surfaces defining an annular opening through which bunches of phase-focused electrons may be emitted, said coaxial surfaces being axially adjustable with respect to each other, tubular anode means having opposite ends and being coaxial with respect to the extended axis of said surfaces, one end of said anode means being disposed adjacent to said opening to provide an electron-accelerating field, means including said anode means and said surfaces for forming electrons emitted from said opening into a beam of spaced electron bunches which converges toward said axis, and means for generating an electrical signal in response to interception of said beam.

26. The electron discharge device of claim 25 wherein the first-mentioned coaxial surfaces in cross-section are concave facing each other and are relatively adjustable to a position at which said surface extension can extend axially beyond the inner annular surfaces in a direction toward said anode means.

27. The electron discharge device of claim 25 wherein the first-mentioned coaxial surfaces in cross-section are concave facing each other and are relatively adjustable to a position at which said surface extension can extend axially beyond the inner annular surfaces in a direction toward said anode means, said inner annular surface including a peripheral portion of one of said electrodes which is cylindrical in shape, said one electrode having a flat normal to said axis and disposed opposite said anode means.

28. The electron discharge device of claim 25 wherein said field-applying means is a resonant cavity and said surfaces constitute capacitive parts thereof.

29. An electron discharge device comprising first means for providing a multipacting volume, second means for applying an electronmultipacting RF field across said volume, said first means providing an electron-emitting opening communication with said volume, means external to said volume for establishing a positive potential which extends through said opening into said volume, said positive potential increasing in a direction extending from within said volume through said opening and beyond the latter, whereby electrons gene-rated within said volume will be emitted through said opening in bunches produced by phase-focusing and quasi-phase-focusing.

30. An electron discharge device comprising means providing a multipacting gap, said means including two spaced apart facing surfaces which define said gap therebetween, said surfaces also defining an electron-emitting opening which communicates with said gap, means for applying a variable electric field to said surfaces and across said gap, said field being such as to produce phasefocusing in said gap, means external to said gap for establishing positive potential which extends through said opening into said gap, said positive potential increasing in a direction extending from within said gap through said opening and beyond the latter, said positive potential also being such as to produce in cooperation with said variable field quasi-phase-focusing of electrons in said gap.

31. An electron discharge device of claim 30 wherein said positive potential means defines a space through which electrons which emanate from said opening may travel, and means including said positive potential means for directing said electrons through said space and for generating an electrical signal in response thereto.

References Cited by the Examiner UNITED STATES PATENTS Re. 22,580 12/1944 Mouromsteff et a1. 3 l55.37 X 2,200,745 5/1940 Heymann 313-253 X 2,404,417 7/ 1946 Varela 3 l3l04 X 2,939,991 6/1960 Beck 3l5-5.38 X 2,967,260 1/1961 Eitel 315-537 ELI LIEBERMAN, Primary Examiner.

HERMAN KARL SAALBACH, R. D. COHN,

Assistant Examiners. 

1. AN ELECTRON DISCHARGE DEVICE COMPRISING CATHODE MEANS FOR EMITTING SPACED BUNCHES OF ELECTRONS IN A GIVEN DIRECTION, MEANS FR ACCELERATING SAID BUNCHES IN BEAMLIKE FORM IN SAID DIRECTION ALONG A PREDETERMINED PENCILLIKE PATH, AND CATCHER MEANS FOR ABSORBING ENERGY FROM ALL OF SAID BUNCHES AFTER ONE TRAVERSAL OF SAID PATH, AND 